I've experienced others. Newer phone? I'm guessing higher resolution pic from a better camera. If the site allows unlimited size and numbers of photos they could quickly run into an expensive hosting situation.
In Windows I have an app so I just right click on a photo in its file folder and select "resize" then choose the resolution...I can either replace the original or change the name and have a duplicate.
On Android phone, i open a separate app called Photo & Picture Resizer (Free version). Similar except I need to open that app on the phone vs just the file folder in windows. I'd be very surprised if there isn't something available on iOS doing the same thing.
All it does is decrease file size, doesn't crop or anything. Frankly the quality difference is not noticable when it's done. Sure, it wouldn't print as well or you cannozoom in as much, but it works great for posting pics on a forum. About any photo I've post here has been resized.